To set the scene and let the audience know what context this play is taking place in and also to foreshadow the events of the story.
If there was no prologue it would not be hard to see that the Capulets and Montagues are fighting but it still exists to tell us that the families have an ancient grudge. It is also vaguely letting the audience know that it does not end well for the lovers. The prologue alerts us of setting (In Fair Verona) alerts us of the characters relationships with each other and things like that.
Overall the play could stand without its prologues but they are still relevant and important for helping people understand.